1. Field of the Invention
This invention relates to an image processing method and a computer readable medium for image processing for visualizing volume data.
2. Description of the Related Art
A revolution is brought about in the medical field with the advent of a CT (Computed Tomography) apparatus and an MRI (Magnetic Resonance Imaging) apparatus making it possible to directly observe the internal structure of a human body as the image processing technology using a computer moves forward, and medical diagnosis using the tomographic image of a living body is widely conducted. While, three-dimensional structure inside of a human body is hard to understand simply from the tomographic image of a human body. Further, in recent years, as a technology for visualizing the complicated three-dimensional structure has advanced, for example, volume rendering for directly rendering an image of the three-dimensional structure from three-dimensional digital data of an object provided by a CT apparatus has been used for medical diagnosis.
MPR (Multi Planar Reconstruction) for cutting out an &arbitrary cross section from volume data and displaying the cross section, CPR (Curved Planer Reconstruction) for setting a path in volume data, setting a cross-sectional curved surface along the path, and displaying the cross-sectional image, MPR with thickness for cutting out a region with some thickness and performing MPR processing to reduce noise and make meandering tissue such as a vessel easy to observe, and the like are generally used as three-dimensional image processing in the volume rendering. MPR with thickness can be obtained b combining plural of parallel cross section images. Further, a term “flat MPR” will be used to emphasize that MPR is related with flat surface.
On the other hand, a projection method of setting an eye point and a projection plane and projecting volume data existing between the eye point and the projection _plane onto the projection plane as seen from the eye point is available. A parallel projection method and a perspective projection method are known as the projection method.
The parallel projection method is a method of setting an eye point at an infinite distance and projecting volume data onto a projection plane in parallel from the eye point; the method is suited for forming the object such as an organ converted into volume data as an image viewed from the outside of the object. On the other hand, the perspective projection method is a method of projecting volume data existing between an eye point and a projection plane onto the projection plane radially with the eye point as the center position; the method is suited for creating a virtual endoscope image for an organ.
FIGS. 18A-18C are drawings to describe flat MPR in a related art for generating a cross-sectional image of Volume data 61. The flat MPR is a technique, for example, for cutting the volume data 61 containing an intestine 62 by a cross section 63 of an; arbitrary plane as shown in FIG. 18A and FIG. 18B, and rendering voxels on the cross section 63 as shown in FIG. 18C, thereby providing a cross-sectional image 64 of the intestine 62 as shown in FIG. 18C.
In the flat MPR, only the tissue on a single plane is rendered and the 3 dimensional relation of the tissue is hard to understand and thus a method of cutting volume data by two or more planes crossing each other, exfoliating the cut planes, and displaying a plurality of MPRs at the same time. For example, exfoliating and displaying the cross section cut by a cube is known. (For example, refer to JP-A-11-219448.)
A method of projecting light onto a spherical surface from the inside with the center of a sphere as an eye point for volume data and exfoliating the spherical projection image to a plane and displaying the image is known. In this case, the spherical surface is approximated to a polyhedron (cube/26-plane polyhedron) and light is projected perpendicularly onto each plane. (For example, refer to U.S. Pat. No. 6,369,812.)
FIG. 19 is a drawing to describe virtual endoscope image in a related art n the virtual endoscope image in the related art, an intestine 62 is seen from an eye point 66 and a perspective projection image 67 projected onto a projection plane 64 is displayed. Thus, to observe the intestine 62, only the medial surface of the intestine 62 is displayed.
FIGS. 20A-20C are drawings to describe problem (1) of the flat MPR in the related art. In the flat MPR in the related art, if a cross section 70 is perpendicular to the running direction of an intestine 62, the whole shape of the intestine 62 becomes clear according to a cross-sectional image 71 as shown in FIG. 20B; if the running direction of the intestine 62 is inclined with respect to cross sections 68 and 72, the perpendicular cross section of the intestine 62 is not displayed as shown in cross-sectional images 69 and 73 and thus the cross-sectional shape of the intestine 62 is hard to understand, as shown in FIG. 20A and FIG. 20C. If the angle of the cross section 68, 70, 72 is changed, the location of the cross section moves and thus it is difficult to understand the displayed part of the intestine 62.
On the other hand, in FIG. 20B, the cross section of the intestine 62 displayed in the cross-sectional image 71 is almost circular and therefore it can be determined that the cross section 70 is orthogonal to the intestine 62; however, if an organ is not circular, it is difficult to determine the orientation of the cross section. Particularly, to conduct a precise diagnosis, periphery of the affected part should be observed based on the orthogonal cross section, and thus if the orthogonal cross section cannot be determined from an MPR image, smooth medical examination is hindered.
FIG. 21 is a drawing to describe problem (2) of the flat MPR in the related art. To conduct virtual endoscope inspection on tubular tissue such as an intestine 62, hitherto a center path 74 of the tubular tissue has been set using MPR images in cross sections 1 to 7, etc. However, since it is hard to understand the depth and the running direction of the tubular tissue, it is difficult to set an appropriate orthogonal cross section angle.
That is, to obtain a precise image, it is necessary to set a cross section perpendicular to the running direction of the virtual endoscope inspection, but it is difficult to adjust the angle of the cross section to meandering of the tubular tissue. For example, the center path 74 is determined while viewing the MPB images in cross sections 1 to 7, etc.; to make a transition from cross section 3 to cross section 4 or from cross section 5 to cross section 6, it is difficult to find out the center of the intestine. Further, it is also difficult to set the center path connecting the tissue centers.